Container shipping is different from conventional shipping because it uses containers of various standard sizes—20 foot (6.09 m), 40 foot (12.18 m), 45 foot (13.7 m), 48 foot (14.6 m), and 53 foot (16.15 m)—to load, transport, and unload goods. Their internal lengths are about eight to ten inches less than the container's length. These containers are often eight feet wide by eight and a half feet tall, and their internal width is usually seven feet, seven inches and internal height is seven feet, nine inches. As a result of the standard, sizes, containers can be moved seamlessly between ships, trucks and trains. The two most important, and most commonly used sizes today, are the 20-foot and 40-foot lengths. The 20-foot container, referred to as a Twenty-foot Equivalent Unit (TEU) became the industry standard reference so now cargo volume and vessel capacity are commonly measured in TEU. The 40-foot length container—literally two TEU—became known as the Forty-foot Equivalent Unit (FEU) and is the most frequently used container today.
The container sizes need to be standardized so that the containers can be most efficiently stacked—literally, one on top of the other—and so that ships, trains, trucks and cranes at the ports can be specially fitted or built to a single size specification. This standardization now applies across the global industry, thanks to the work of the International Organization for Standardization (ISO) that in 1961, set standard sizes for all cargo containers.
Containers undergo loading and unloading of many types of material, such as scrap metal, that risk damage to the side walls of the container when the material is moved in-bulk into the container.